This invention relates to a process for the production of moldings of polyurethane foams having a compact surface wherein special organic carboxylic acids are used as blowing agents.
The production of moldings of polyurethane foams having a compact surface by in-mold foaming is known in principle (see, for example, German Auslegeschrift 1,196,864). It is carried out by foaming a reactive and foamable mixture of organic polyisocyanates, compounds containing isocyanate-reactive groups and the usual auxiliaries and additives in molds, wherein a larger quantity of the reaction mixture is introduced into the mold than is required for filling the mold by free foaming. It is possible, by suitable selection the starting components, more particularly in regard to their molecular weight and their functionality, to produce both flexible, semirigid, and rigid moldings. The compact outer skin is obtained on the one hand by introducing more foamable mixture into the mold than is required to fill the mold by free foaming and, on the other hand, by using blowing agents such as, for example, fluorocarbons, which condense on the inner wall of the mold under the prevailing temperature and pressure conditions so that the blowing reaction comes to a stop on the inner wall of the mold and a compact surface skin is formed.
In addition to water, chlorofluorocarbons or hydrogen chlorofluorocarbons are used as blowing agents in the production of polyurethane (PUR) foams. These conventional blowing agents have enabled a number of favorable properties to be obtained. In particular, fluorotrichloromethane enables a number of foams differing in their properties to be produced. However, the group of so-called chlorofluorocarbon blowing agents are problematical in ecological terms.
Attempts have been made to switch entirely to water as an alternative blowing agent. However, the use of water as a blowing agent for semirigid to flexible PUR foams with a compact outer skin presents problems which impose narrow limits on the use of water. Serious disadvantages for the satisfactory processing of water-blown moldings include flow behavior, closed cell structure, and the structure of the surface skin.
Problems arise with respect to flow behavior as the additional water greatly accelerates the increase in viscosity, so that the mixture of raw materials loses the necessary mobility. Accordingly, correspondingly large quantities of the reaction mixture have to be used to fill the mold.
The water-blown moldings have a tendency to develop a pronounced closed cell structure. This results in the buildup of pressure and ultimately in splitting of the moldings. For these reasons, narrow limits are imposed on the density of water-blown semirigid moldings. Thus, densities of only 500 to 600 kg/m.sup.3 may be regarded as critical on account of the hereinabove described effects, with water having been used in a quantity of 0.6 to 0.8 part by weight.
Problems are also seen in the structure of the compact surface skin when water is used as the blowing agent. In particular, water-blown semirigid moldings do not have a cell-free surface skin, so that an additional lacquer coating is required for practical applications.
Numerous blowing agents have previously been recommended. Thus, French Patent 2,509,317 uses formic acid and water for the production of flexible slabstock foams and rigid foams. European Patent Application 0,154,669 uses special amines and acids (e.g. formic acid) for the production of flexible foams. According to European Patent Application 372,292, carboxylic acids, preferably lactic acid, are used as blowing agents for the production of rigid foams having a compact surface skin.
These more recent, known blowing agents should be at least as good in their properties as the monofiuorotrichloromethane previously used, and should form correspondingly good foams. Accordingly, the blowing agents must satisfy the following requirements:
a) high gas yield corresponding to a low density PA1 b) problem-free flow PA1 c) minimal increase in viscosity in the foaming reaction mixture PA1 d) dimensionally stable moldings PA1 e) high compression gradient without splitting PA1 f) void-free surface PA1 g) rapid reduction in pressure in the molding PA1 h) safe decomposition products. PA1 a) a polyisocyanate component comprising at least one aromatic polyisocyanate, PA1 b) an isocyanate-reactive component comprising at least one organic compound containing at least two isocyanate-reactive groups, PA1 c) urethane- and urea-forming activators, PA1 d) organic blowing agents PA1 wherein: R represent a CH.sub.2 group, or a CH.dbd.CH group; PA1 e) other auxiliaries and additives, and PA1 f) water.
For various reasons, the solutions known from the prior art do not work for certain types of foams.
Thus, the formic acid used as blowing agent has a chain-terminating character and forms carbon monoxide as a decomposition product.
The blowing agents known from European Patent Application 372,292 present problems in the production of flexible moldings having a compact surface skin.
Accordingly, the problem addressed by the present invention was to provide a process which would enable moldings of polyurethane foams with a compact surface skin to be produced and which would not have any of the disadvantages mentioned hereinabove. This problem has been solved by the process according to the invention wherein certain organic carboxylic acids are used as blowing agents.